Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
A clear trend in the field of wind energy is to increase the size of the wind turbines. Both hub height and blade length have increased dramatically in recent years, and it appears that this trend will continue. With an increase in blade length, blades that are relatively lighter are needed in order to control the Cost of Energy (COE) of the wind turbine.
As blades become lighter, blade loads such as e.g. the blade root bending moment may become more critical. The blade loads may be substantially affected by e.g. wind gusts, turbulence and wind shear. Changing wind velocities, and more particularly quickly changing wind velocities can especially affect the blade loads, and thereby impact the fatigue of various wind turbine components, such as e.g. the blades, hub, rotor shaft or gearbox.
Pitch systems may be employed in wind turbines for adapting the position of a wind turbine blade to varying wind conditions by rotating the blade along its longitudinal axis. In this respect, it is known to rotate a wind turbine blade in such a way that it generates less lift (and drag) when the wind speed increases. This way, even though the wind speed increases, the torque transmitted by the rotor to the generator remains substantially the same. It is furthermore also known to rotate wind turbine blades towards their stall position (so as to reduce the lift on the blades) when the wind speed increases. These wind turbines are sometimes referred to as “active-stall” wind turbines. Pitching may furthermore also be used for rotation of the blade towards its vane position, when a turbine is temporarily stopped or taken out of operation for e.g. maintenance.
A common control strategy of a variable speed wind turbine is to maintain the blade in a predefined “below rated pitch position” at wind speeds equal to or below nominal wind speed (for example from approximately 3 or 4 m/s to 10 m/s). Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions depends however on the complete design of the wind turbine. In the lower wind speed regions (at “partial load”), the objective is generally to maximize power output by maintaining pitch constant, thereby catching maximum energy, and varying generator torque and the rotor speed to keep the power coefficient, Cp, at a maximum. Above the nominal speed (for example from approximately 10 m/s to 25 m/s), the blades are rotated to maintain the aerodynamic torque delivered by the rotor substantially constant. This means that the angle of attach of the blades is generally reduced with increasing wind speed. Cut-in wind speed may e.g be around 3 m/s, nominal wind speed may be e.g. around 10 m/s and cut-out wind speed may e.g. be around 25 m/s. The nominal wind speed, cut-in wind speed and cut-out wind speed may of course vary depending on the wind turbine design.
Furthermore, aerodynamic devices such as e.g. flaps, slats, spoilers, and Boundary Layer Control devices may be employed on blades of wind turbines in order to influence and control the aerodynamic flow around the blades. Flaps along the trailing edge of a blade can be actuated relatively quickly and have the capability of quickly modifying the lift and drag (loads) of a blade. Flaps may therefore be employed on larger wind turbine blades to quickly alleviate loads when needed.
In wind turbines wherein the blades comprise one or more flaps, and wherein the blades may be rotated substantially along their longitudinal axes by a pitch mechanism, a joint control of the various actuators is needed in order to make sure e.g. that loads stay within predefined limits, fatigue is limited and electrical power may be generated efficiently (at a relatively low Cost Of Energy).